Soda ash is the active ingredient in washing soda. The chemical name for it is sodium carbonate, chemical formula Na2CO3. It is more basic, that is, less acidic, than sodium bicarbonate (baking soda), whose chemical formula is NaHCO3. The purpose of sodium carbonate is simply to increase pH.
Some forms of soda ash (e.g. that labeled as 'washing soda') contain more water molecules than others, which makes them weigh more and be larger for a given number of sodium carbonate molecules - this means that you need to measure out a larger quantity of the hydrated form in order to get the same results. If you buy sodium carbonate without the extra water molecules, then store it for several years in humid conditions, it will absorb some water and appear to lose strength, when in fact it has merely 'bulked up' and needs to be used in larger volumes.
The type of sodium carbonate used in washing soda is a decahydrate, so you need to use a lot more washing soda than you would anhydrous soda ash, perhaps three times as much - assuming that you are able to find pure unadulterated washing soda that is suitable for use in dyeing. (In theory, we should use 2.7 times as much washing soda as a substitute for soda ash, if measuring by weight, or 4.6 times as much if measuring by volume.)
Although you can buy washing soda in the grocery store, this is usually advised against, because some US brands in the past were known to contain optical brighteners, salt, and/or fragrances. However, Arm & Hammer brand "Super Washing Soda" does not now contain any additives, though you must use a larger quantrity due to its greater hydration level (see above). A better source is a swimming pool supplies store, or a hardware store that carries some pool supplies, which will carry pure sodium carbonate (a popular brand is pH Up), sold for the purpose of increasing the pH of pool water. This is typically cheaper than mail-ordering from a dye supplier, and just as good. Beware of suppliers that try to sell you sodium bicarbonate instead of sodium carbonate; bicarbonate is much less basic (it has a lower pH), and will not work for most dyeing recipes.
Soda ash changes the pH of the fiber-reactive dye and cellulose fiber so that the dye reacts with the fiber, making a permanent connection that holds the dye to the fiber. It actually activates the fiber molecules so that they can chemically attack the dye. (It can also be used with silk, but not other protein fibers such as wool.)
No, soda ash will not make all purpose dye, such as Rit® or Tintex®, permanent on the fabric. Instead, you must either use a washfast dye such as Procion MX dye, which does require soda ash, or use a commercial cationic fixative in order to make all-purpose dye acceptably washfast, so it does not bleed in the laundry forever.
There are three choices, depending on what you're doing: add the soda ash before the dye, add it with the dye, or add it afterwards.
Adding the soda ash before the dye is the usual method for tie-dye. First tie the garments, or leave them loose. (It is all right to put slightly damp garments into the presoak solution.) Make up a solution of 1/2 or 1 cup of soda ash per gallon of water (soda ash dissolves best in warm water, 96°F. or 35°C.), and soak the material from 5 minutes to an hour, depending on your recipe and materials. Then, wearing gloves, wring out the material, and prepare to apply the dye. (Optionally, you may chose to line-dry your soda-ash-presoaked garments, for some dyeing techniques. The soda ash stays in the fabric. Be cautious, as the dust can be irritating to breathe or to the skin.)
Adding the soda ash with the dye is commonly used with dye painting. A possible drawback is the fact that the dyes will retain their ability to dye fabric for only a few hours after the soda ash is mixed into them. (Procion MX dyes without soda ash will stay good for at least a week after mixing, at normal room temperature.)
Adding the soda ash after the dye is the usual method for immersion dyeing and low water immersion dyeing.
In dyeing cotton or any other cellulose fiber, such as linen, rayon, tencel, hemp, etc., the requirement is simply to increase the pH (alkalinity) of the reaction to somewhere around 10.5 to 11; the exact ideal pH depends on the fiber being used and the individual dye color. (See What is the effect of pH?.)
Caustic chemicals. Other chemicals that can, in theory, be used to reach this ideal pH include NaOH (sodium hydroxide, or lye) and TSP (trisodium phosphate, sold as a heavy duty cleaner at hardware stores). Pure NaOH is very caustic and quite dangerous for those not trained as chemists; TSP is somewhat safer, but not as safe for everyday use as soda ash. It is mainly these practical reasons that lead us to recommend the use of soda ash, instead. Another reason is that soda ash will produce close to the correct pH even if you use a little bit too much or too little; too much NaOH will produce a pH that is too high to work well, while too little will produce a much lower pH, again non-optimal. If you wish to substitute any other pH raiser for soda ash, be sure to check the pH of your solution to make sure that it is appropriate. Too low of a pH will not work; too high will damage fibers and possibly endanger you. Don't forget: always add pH adjusting chemicals to water, not the other way around; put water into the container before NaOH or TSP, not afterwards. (This is less important with soda ash, though not a bad idea all the same.)
Sodium silicate. Fabric that has been painted with Procion MX or similar dyes, and the dye allowed to dry, can be treated with a liquid sodium silicate solution, also known as water glass. This product is sold under the name of PRO Fix LHF and PRO QuickFix by PRO Chemical & Dye, as Drimafix by Batik Oetoro, as Tobafix by Tobasign Dyes, and as AfterFix by Dharma Trading Company. Sodium silicate will substitute nicely for the soda ash step above. (See Sources of Supplies page for contact information.) AfterFix is a solution of sodium silicate. One possible drawback is that acidic solutions can turn sodium silicate into a gel that is extremely difficult to remove, so be sure to rinse well with alkaline or neutral water if at any step you introduce a low pH.
Sodium bicarbonate. Baking soda, or sodium bicarbonate, decomposes at sufficiently high temperatures to form sodium carbonate, so it is an ideal substitute for dyes that will be fixed by extensive steaming or baking. It should not be used for room-temperature dyeing. Sodium bicarbonate at room temperature will produce a pH of only around 8, too low for ideal use on cotton and other cellulose fibers, though it is capable of producing some reaction.
Acetic acid or citric acid. Protein fibers such as wool and silk can also be dyed at low pHs, substituting white vinegar (and heat!) for the soda ash. (Wool is damaged by high pHs such as we use for dyeing cellulose.) Do not try low pH (acidic) reactions with cellulose fibers, as they simply do not work.
The FMC Corporation provided a useful technical data sheet on Soda Ash at their web site; see FMC Soda Ash Storage Options abnd Technical Data [PDF]. (Thanks to Sally Holmes for this tip.)
Last updated: January 22, 2015
Page created: November 13, 1999
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